Reflex traveling wave amplifier



NOV. 30, 1954 GINZTQN 2,695,973

REFLEX TRAVELING WAVE AMPLIFIER Filed 001:. 27. 1949 2 Sheets-Sheet 1GAIN FREQUENCY INVENT R ATTORNEY 1954 E. L. GINZTON REFLEX TRAVELINGWAVE AMPLIFIER 2 Sheets-Sheet 2 Filed Oct. 2'7, 1949 INVENTOR Bow/m LG/NzTo/V ATTORNEY United States Patent Ofiice 2,695,973 Patented Nov.30, 1954 REFLEX TRAVELING WAVE AMPLIFIER Edward L. Ginzton, Menlo Park,Calif., assignor to The Board of Trustees of the Leland Stanford JuniorUniversity, Stanford University, Calif., a legal entity having corporatepowers of California Application October 27, 1949, Serial No. 123,820

9 Claims. (Cl. 315-3) This invention relates to the art of amplificationof high frequency wave energy, and more particularly to improvements intravelling wave amplifiers, wherein an electron stream reacts with atravelling or running wave to increase the amplitude thereof.

Prior travelling wave amplifiers in general operate by projecting astream of electrons in the direction of travel of the wave, but at asomewhat greater velocity. Under proper conditions, the electrons giveup kinetic energy to the wave, which thus grows in amplitude as ittravels with the stream.

According to the present invention, electrons are projected in a streamtransversely of the direction of travel of the wave and the electronstream is bunched or density modulated in such manner as to reinforcethe wave. Reference is made here to copending application Serial No.86,018, filed April 7, 1949, by Edward L. Ginzton and entitled ElectronDischarge Devices.

Said copending application describes a travelling wave amplifierincluding two spaced parallel wave guides or conduits, with means forprojecting a sheet of electrons first through one conduit and. thenthrough the other. Input wave energy applied to the first conduitproduces velocity modulation and consequent bunching of the electronstream, which induces an amplified travelling wave in the secondconduit. The process is continuous throughout the active length of theconduits, causing the induced wave to grow in amplitude as it travels.

The present invention deals with amplifiers which are similar in somerespects to those of said copending application, but involve a singlewave conduit, with input energy applied to one end and output energytaken from the other. The electron stream passes twice across theconduit, being velocity modulated by the first interaction with thewave, and delivering energy to the wave by the second interaction.

It is one of the principal objects of this invention to providetravelling wave amplifiers which can be designed to give substantialvoltage gain at microwave frequencies, at any desired frequencythroughout a relatively wide band.

Another object is to provide travelling wave amplifiers utilizing theprinciple of reflection bunching, whereby a single wave guide or conduitof reasonably short length may be used to achieve a desired degree ofamplification.

A further object is to provide a type of travelling wave amplifier whichis simple to design and requires a minimum of precision mechanicalconstruction.

One of the more specific objects of the invention is to provide methodsand means for causing ubstantially unilateral amplification in a deviceof the described type.

The invention will be described with reference to the accompanyingdrawings, wherein:

Fig. 1 is a longitudinal section of a travelling wave amplifierembodying the invention,

Fig. 2 is a cross section of Fig. 1 in the plane 2--2,

Fig. 3 is a graph showing typical gain vs. frequency characteristics ofa device like that of Fig. i,

Fig. 4 is a perspective view in section of a modification of thestructure of Fig. 1, and

Fig. 5 is a transverse cross section of another modification of thedevice of F ig. 1.

Referring first to Figs. 1 and '2, a wave guide 3 is provided with alongitudinally extending opening or slot 7 along the center of its upperwall. The opening 7 contains an electron permeable conductive grid 9.The lower wall of the guide 3 is vformed with reentrant portions 11which define an Opening 13. parallel to the opening 7. The. upper end ofthe opening 13 is provided with a grid 15 adjacent the grid 9, and thelower end includes a similar grid structure 17.

A strip-like cathode 19 is disposed below and parallel to the grid 17,and is provided with a heater 21 which is connected between a pair ofterminal posts 23 and 23. A focussing electrode 25 extends along bothsides of the cathode 19 and is connected thereto. A reflector orrepeller electrode 27 is supported on a pair of terminal posts 29 aboveand parallel to the upper grid 9. This assembly of elements constitutesan elongated electron gun for projecting a sheet beam of electronstransversely through wave guide 3.

The terminal posts 29, and also the terminal posts 23 and 23 aresupported in insulating seals 31 in the walls of a vacuum-tight envelope33. The envelope 33 surrounds all of the above described structure andis provided with end walls 35, one of which includes an exhaust stem 37through which the envelope is evacuated. The envelope 33 may be made ofmetal, as shown, or of other suitable material such as glass. Insulatingseals 39 are provided in the wave guide 3 in the vicinity of the endwalls 35.

One end of the wave guide 3, for example the left hand end, is adaptedto be coupled to a source (not shown) of high frequency waves which areto be amplified. The other end of the guide 3 is adapted to be coupledto a load for utilization of the amplified waves. A low voltage source,such as a battery 41, is connected between the heater terminal posts 23and 23'. The wave guide 3 is grounded, together with the envelope 33 andany other metallic parts connected to it.

A relatively high voltage source 43 is arranged to maintain the cathodeassembly, which includes the cathode 19 and the focussing electrode 25,at a negative potential with respect to the wave guide 3 and the grid17. A further source 45 is shunted by an adjustable voltage divider 47,Whose movable tap 49 is connected to the repeller electrode 27 throughone of the terminal posts 29. The source 45 and the voltage divider 47are arranged to maintain the repeller 27 at a negative potential ofadjustable magnitude.

In the operation of the system of Figs. 1 and 2, the cathode assemblyproduces a sheet-like stream of elec' trons. The grid 17, being positivewith respect to the cathode, accelerates the electrons to a substantialvelocity. The electric field between the grid 17 and the focussingelectrode 25 is such as to direct the electrons in a relatively thinsheet through the grids 17, 15 and 9 toward the repeller electrode 27.

Input wave energy applied to the guide 3 sets up a travelling wavetherein, so that the instantaneous voltage between two juxtaposed pointson the grids 9 and 15, such as the points 51 and 53, varies cyclicallyat the input frequency. Most of the electrons emerging from theaccelerator grid 17 have substantially the same velocity, and they enterthe grid 15 at that velocity. However, those which cross the gap betweenthe points 51 and 53 at a time when the point 51 is positive withrespect to the point 53 will be further accelerated; those which crossthe gap a fraction of a cycle later, when there is efiectively novoltage between the points 51 and 53, will emerge from the grid 9 attheir original velocity, and those which cross while the point 51 isnegative with respect to the point 53 will be decelerated.

Thus the element of the electron stream which goes through the points 53and 51 is velocity modulated, substantially in the same manner as thebeam in a klystron tube is modulated by the field in its buncherresonator. The electrons emerging from the upper grid 9 approach therepeller 27, but its potential is adjusted to a value such that theirkinetic energy is overcome and they are forced to travel back toward thegrid 9. The more rapidly moving electrons approach the repeller moreclosely before their flight is reversed, just as a ball thrownvertically up with a high velocity will go higher than one thrown with alow velocity. Similarly, the high velocity electrons will take longer toreturn to the grid 9, just as a fast ball will take longer to return tothe ground than a slow one.

As a result of the difference between the times of flight of the fastand slow electrons, a slow electron will reach a certain point in thereturn path at the same time as a fast electron which emerged earlierfrom the grid 9. Thus the velocity modulated stream becomes bunched ordensity modulated, since each electron which crosses the gap betweenpoints 53 and 51 without change in velocity tends to become the centerof a cloud of electrons which have emerged earlier at higher velocityand later at lower velocity.

The potential of the repeller 27 is adjusted to make the bunching occurin the vicinity of the points 51 and 53. [t is also necessary to makethe relationship between the repeller and accelerating voltages suchthat each bunch will cross the gap at a time when it will reinforce thewave travelling along the guide. In order to do this, it is found thatthe bunch must arrive slightly in advance of a voltage maximum betweenthe points 51 and 53, when the gradient is in such direction as toabsorb energy from the electrons, i. e. when the point 51 is positivewith respect to the point 53.

In making the return trip from the point 51 to the point 53, theelectrons give up part of their kinetic energy and emerge from the lowergrid 53 at a relatively low velocity. They are collected by thereentrant walls 11 and other conductive parts of the structure. Like theelement 1 of the stream which goes through the points 53 and 51, eachother element is modulated, bunched, and delivers energy to the wave,the wave being operated upon in succession by the successive streamelements as it travels along the guide.

Viewed in one aspect, the bunched stream acts like a uniform admittancesheet across the guide, in which the conductance component is negative.The wave amplitude increases exponentially, since the greater itsamplitude as it reaches any given part of the guide, the greater is thebunching and the more is its amplitude increased. However, the actualvoltage amplitude that can be attained is limited by the acceleratingvoltage, because when the amplitude of the voltage between the grids 53and 51 becomes high enough, the returning bunched electrons will nothave suflicient kinetic energy to re-cross the gap, and thus cannot addany energy to the wave.

An important point to be noted in the operation of the system of Fig. lis that since the structure is electrically symmetrical, it will amplifywaves travelling in one direction as well as waves travelling in theother. While this characteristic might be advantageous under somecircumstances, it makes it necessahy to provide a good impedance matchbetween the ends of the guide 3 and the respective source and loadelements which are coupled thereto. The reason for this is that anysubstantial reflections will set up a standing wave in the guide, andthe standing wave will be amplified in both directions, producingsustained oscillation of the system.

Although the wave guide 3 may be matched at both ends, and thereforeoperate as a non-resonant, i. e. periodic device, it will be found thatany particular adjustment of the repeller and accelerator voltages willprovide amplifica' tion only at certain frequencies, rather thancontinuously throughout the wide pass band of the wave guide. This isbecause the average travel time of the electrons in the space betweenthe grid 9 and the repeller must bear a definite relationship to thefrequency, i. e. the electron bunches must be made to arrive in theproper phase relationship with the wave. Since the travel time may beapproximately one, two, three, or any other small number of cycles, oneset of adjustments will provide amplification throughout severaldiscrete frequency bands, as shown in Fig. 3.

As a result of the wide distribution of the gain vs. frequencycharacteristic, the wave guide in Fig. 1 must be matched not only at theparticular frequency where amplification is desired, but also at all theother frequencies where substantial amplification can occur, ifuncontrolled oscillation is to be prevented. In view of this ratherstringent requirement, it may be preferable to employ a modification ofthe structure of Fig. 1, including means to make the device unilateral,providing amplification of waves travelling one way and attenuation ofwaves travelling the other way.

The desired unilateralamplification characteristic may be achieved bygiving the electrons a velocity component which is in the direction ofwave propagation in the wave guide. Then, with the accelerator andrepeller voltages adjusted to make the electron bunches arrive at theguide in the proper phase to reinforce a wave travelling in onedirection, a wave travelling in the opposite direction will beattenuated. a

Referring to Fig. 4 the structure of Fig. 1 is modified by making theelectron gun assembly 19, 25 and the grid 17 diverge from the wave guide3 in the direction of propagation in which amplification is to occur.The axes of the elements remain in a common plane, as in Fig. l. Thearrangement of Fig. 4 causes the electrons to follow paths crossing thegap between the grids 9 and 15 at one point anddrecrossing at anotherpoint which is further along the gut e.

The electron trajectories may be controlled in the required manner forunilateral amplification by means of a tranverse magnetic field, asshown in Fig. 5. Here a pair of magnetic pole pieces 57 and 59 extendlengthwise of the structure, facing each other between the cathode 19and the lower side of the wave guide 3. The poles 57 and 59 may beenergized by a suitable winding, not shown. Although the acceleratinggrid may be arranged in the same way as in Fig. 1, it may be preferableto extend the walls 11 below the guide as at 11', placing theaccelerating grid 17 below the pole pieces 57 and 59.

In the operation of the system of Fig. 5, the electrons are firstaccelerated upwardly, i. e. directly toward the grid 17, and they emergetherefrom at substantially uniform velocity. The transverse fieldbetween the poles 57 and 59 deflects the electrons in the direction ofwave propagation along the guide, so that bunches formed by the field inone part of the guide will return to and reinforce the field in anotherpart further along the guide.

It will be evident without further illustration that other arrangements,for example combinations of the systems shown in Figs. 4 and 5, may beused to impart a longitudinal velocity component to the electron streamfor obtaining unilateral amplification.

Since many changes could be made in the above construction and manyapparently widely different embodiments of the invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A reflex travelling wave amplifier, including an aperiodic waveconduit having input and output openings at its respective ends andhaving two opposed substantially parallel electron permeablelongitudinal walls, the distance between said walls being very shortcompared to their length to define a gap that is narrow laterally ofsaid conduit but wide longitudinally of said conduit, the electricvector of energy propagated by said conduit extending between saidlongitudinal walls, means outside said conduit and adjacent one of saidwalls and substantially longitudinally coextensive with said gap forproducing and projecting a flat sheet stream of electrons across saidgap, and means outside said conduit and adjacent the other of said wallsfor changing the direction of flow of said electrons after they crosssaid conduit to make them recross said gap in substantially the oppositedirection.

2. In a unilateral wave guide amplifier, a wave guide having an inputopening at one of its ends and an output opening at the other of itsends, the length of said guide being substantially greater than itsmajor cross sectional dimension, an electron gun outside and adjacentsaid wave guide and substantially longitudinally coextensive therewithfor producing a flat sheet stream of electrons and projecting saidelectrons across said wave guide, said gun being inclined with respectto the axis of said guide to impart said electrons with a component ofvelocity parallel to the axis of said wave guide, and a reflectorelectrode outside said wave guide and on the opposite side thereof fromsaid electron gun for reflecting said electrons to recross said waveguide.

3. In a reflex travelling wave amplifier, an aperiodic hollow waveconductor provided with an input opening at one of its ends and anoutput opening at the other of its ends, said hollow conductor havingtwo opposed electron permeable longitudinal walls defining a gap that isshort transversely of said conductor relative to its lengthlongitudinally of said conductor, means for projecting an electron beamacross said gap with a component of velocity parallel to thelongitudinal axis of said conductor, and means for reflecting saidelectron beam for a return transit across said gap with a substantiallyequal component of velocity parallel to said axis.

4. An amplifier comprising means including a strip cathode and conformalfocusing and accelerating electrodes for producing a substantiallyplanar sheet electron beam, means including opposed magnetic pole pieceson opposite sides of the plane of said beam and extending along saidplane substantially parallel to said cathode for providing a magneticfield directed at right angles to the plane of said beam, meansincluding an electron permeable wave guide in the path of said beam withits longitudinal axis in said plane and substantially parallel to saidcathode, said guide having an input opening at one of its ends adaptedto be coupled to an external source of high fre quency energy forproviding an alternating electromagnetic field with its electric vectorparallel to a component of the velocity of electrons in said beam, andmeans including an electrode adjacent the side of said guide oppositesaid cathode and substantially coextensive with said cathode forproviding an electrostatic field for reflecting said ieilectrons toretraverse said alternating electromagnetic old.

5. In an amplifier, an aperiodic wave conduit having electron permeablewalls substantially longer than the major cross sectional dimension ofsaid conduit, said conduit having an input opening at one of its endsand an output opening at the other of its ends, said input opening beingadapted to be connected to a source of high frequency energy forexciting a travelling electromagnetic wave in said conduit, meansincluding an electron gun adjacent one of said walls and substantiallylongitudinally coextensive therewith for producing a sheet stream ofelectrons and projecting said stream through said walls, each linealelement of said sheet crossing the longitudinal axis of said conduit ata respective first point, and means including a reflector electrodeadjacent said conduit on the side remote from said electron gun, fordirecting said electron stream to make each said lineal element recrossthe longitudinal axis of said conduit at a second respective oint. p 6.An electron discharge device including an aperiodic wave conduit havingopposed longitudinally extending electron permeable wall portionssubstantially longer than the major cross sectional dimension of saidconduit, means including an input aperture at one end of said conduitfor applying wave energy to be amplified thereto, means including anoutput aperture for leading away energy from the other end of saidconduit, a strip cathode and a focusing electrode extending along saidconduit adjacent and substantially longitudinally coextensive with oneof said walls for producing a sheet-like stream of electrons andprojecting said stream through said electron permeable wall portions insuccession in a direction transverse to the direction of wavepropagation in said wave conduit,

and an electron repeller electrode spaced from and generally parallel tosaid conduit and on the opposite side thereof from said cathode, forreversing the flow of said electrons to make said stream re-traversesaid conduit.

7. An electron discharge device including an aperiodic wave conduithaving opposed longitudinally extending electron permeable wall portionssubstantially longer than the major cross sectional dimension of saidconduit, an input aperture at one end of said conduit for applying waveenergy to be amplified thereto, means including an output aperture forleading away energy from the other end of said conduit, a strip cathodeand a focusing electrode adjacent one of said wall portions andsubstantially longitudinally coextensive therewith for producing asheet-like stream of electrons and projecting said stream through saidelectron permeable wall portions in succession in a direction transverseto the direction of wave propagation in said wave conduit, an electronreflector electrode spaced from and generally parallel to said conduitand on the opposite side thereof from said cathode, for reversing theflow of said electrons to make said stream re-traverse said conduit, andmeans between said cathode and said conduit for deflecting saidelectrons in a direction parallel to that of wave propagation in saidconduit, whereby an electron crossing said conduit initially at onepoint will recross it at another point.

8. The invention set forth in claim 7, wherein said last mentioned meansincludes opposed magnetic pole pieces on opposite sides of the plane ofsaid beam and extending along said plane substantially parallel to saidcathode.

9. A reflex travelling wave amplifier, including an aperiodic wave guidewith two opposed substantially parallel longitudinally extendingelectron permeable walls, said wave guide having an opening at one ofits ends adapted to be coupled to a source of high frequency Waves to beamplified, and an opening at its other end adapted to be coupled toutilization means, the electric field of said waves extending betweensaid walls, a strip cathode substantially longer than the major crosssectional dimension of said wave guide and adjacent one of said electronpermeable Walls and longitudinally coextensive therewith substantiallythroughout the length of said cathode, and a reflector electrodeapproximately the same length as said cathode, adjacent the other ofsaid electron permeable walls and longitudinally coextensive with thewave guide throughout the length of said reflector electrode.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,122,538 Potter July 5, 1938 2,153,728 Southworth Apr. 11,1939 2,241,976 Blewett et al. May 13, 1941 2,367,295 Llewellyl Jan. 16,1945 2,450,026 Tomlin Sept. 28, 1948 2,452,075 Smith Oct. 26, 19482,462,085 Fremlin Feb. 22, 1949 2,468,152 Woodyard Apr. 26, 19492,472,038 Yando May 31, 1949 2,509,374 Sunstein May 30, 1950 2,579,654Derby Dec. 25, 1951

